The present invention relates to certain nonionic oligomeric surfactants, in particular such surfactants based on vinyl systems having an amido function, especially acrylamide, methacrylamide or a derivative thereof, and to the use thereof as dispersants and/or stabilisers for hydrophobic materials, especially hydrophobic addenda that are utilised in the manufacture of photographic products.
Micro-precipitated dispersions of photographic couplers, prepared by solvent and/or pH shift techniques are becoming more widely used (see references: UK Pat. No. 1,193,349 by J. A. Townsley and R. Trunley; Research Disclosure 16468, December 1977, pp 75-80 by W. J. Priest; U.S. Pat No. 4,957,857 by K. Chari; U.S. Pat. Nos. 4,970,139 and 5,089,380 by P. Bagchi; U.S. Pat No. 5,008,179 by K. Chari, W. A. Bowman and B. Thomas; U.S. Pat. No. 5,104,776 by P. Bagchi and S. J. Sargeant) and offer benefits in decreased droplet size and often increased reactivity relative to conventional oil-in-water homogenised dispersions. European Patent Publication 0,674,221A (Application No. 95200377.0) (Kodak Ltd.) teaches that increased coupler reactivity leads to higher maximum densities (Dmax) with these dispersions. However, it would be desirable to lower the viscosity of such dispersions, relative to the viscosity obtained with conventional anionic dispersants, and to avoid the problem of flocculation that is usually encountered in gelatin solutions when using typical nonionic dispersants.
Dispersions of solid particle dyes are typically obtained by milling the powdered dye in a ball mill with a dispersant and stabiliser for periods of days. During this time the dyes characteristically mill down to a mean particle size of 50 to 200 nm. The stability of dispersions is habitually tested by subjecting the systems to an accelerated keeping test. This is commonly engineered by holding the systems at an elevated temperature for long periods of time and monitoring processes such as particle growth or flocculation. Typically, solid particle dyes require both a dispersant and stabiliser to form a stable dispersion. Clearly, it would be an advantage if a dispersant could be used that also acted as a stabiliser.
Solid particle cationic sensitising dye dispersions are typically obtained by milling the powdered dye in a ball mill with a dispersant and stabiliser for long periods in a similar manner to the filter dyes. Perhaps owing to their cationic nature, the dyes do not mill well with anionic dispersants or nonionic/ anionic copolymer dispersants, the systems tending to thicken or set up. However, and perhaps more surprising, they do not mill very well with nonionic oligomeric or polymeric dispersants either, the systems still tending to exhibit high viscosity or setting up. Plainly, it would be advantageous to use a dispersant that permitted milling with less thickening or setting up.
The invention, in one aspect, provides hydrophobically capped oligomeric acrylamides and pyrrolidones, in particular the compounds of the general formula
Rxe2x80x94Lxe2x80x94T xe2x80x83xe2x80x83(1) 
or 
in which
R, R1 and R2 are each independently selected from aliphatic hydrocarbyl, aryl-(aliphatic hydrocarbyl) and (aliphatic hydrocarbyl)-aryl groups,
L is a sulphur atom or a linking group that contains a sulphur atom that is attached to the T group, and
T is a hydrophilic oligomeric group obtained by the oligomerisation of one or more vinyl monomers having an amido function.
The present invention also provides, in another aspect thereof, a composition that comprises an aqueous medium containing a hydrophilic colloid, a hydrophobic material dispersed in the medium and a dispersant, wherein the said dispersant is a compound of the present invention as defined above.
The present invention, in yet another aspect, provides a method for making a composition according to this invention, which method comprises dispersing a hydrophobic material in an aqueous medium that is liquid and that contains a hydrophilic colloid, wherein the dispersing is carried out in the presence of a compound of the present invention as defined above.
The present invention further provides a process for producing a photographic material, which process comprises applying to a support a plurality of aqueous layers each of which contains a hydrophilic colloid, wherein at least one such layer comprises a composition of the present invention. Also provided are photographic materials obtainable by this process.
The present invention, in yet a further aspect thereof, provides the use of a compound of the present invention, as defined above, as a dispersant for a hydrophobic material in an aqueous medium that contains a hydrophilic colloid.